Fuel ethanol is currently produced from feedstocks such as cornstarch, sugar cane, and sugar beets. However, the production of ethanol from these sources cannot expand much further due to limited farmland suitable for the production of such crops and competing interests with the human and animal food chain. The use of fossil fuels, with the associated release of carbon dioxide and other products, in the conversion process is a negative environmental impact of the use of these feedstocks
The production of fuel ethanol from cellulose-containing feedstocks, such as agricultural wastes, grasses, forestry wastes, and sugar processing residues has received much attention due to the availability of large amounts of these inexpensive feedstocks and the cleanliness of ethanol as a fuel compared to gasoline. In addition, a byproduct of the cellulose conversion process, lignin, can be used as a fuel to power the cellulose conversion process, thereby avoiding the use of fossil fuels. Studies have shown that, taking the entire cycle into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The cellulosic feedstocks that may be used for ethanol production include agricultural wastes such as corn stover, wheat straw, barley straw, canola straw, and soybean stover. Grasses such as switch grass, miscanthus, cord grass, and reed canary grass may also be used.
Cellulose consists of a crystalline structure that is very resistant to breakdown, as is hemicellulose, the second most prevalent component of these feedstocks. The conversion of cellulosic fibers to ethanol requires liberating cellulose and hemicellulose from lignin or increasing the accessibility of cellulose and hemicellulose within the cellulosic feedstock to cellulase enzymes, depolymerizing hemicellulose and cellulose carbohydrate polymers to free sugars and, fermenting the mixed hexose and pentose sugars to ethanol.
The feedstock is conveyed into the plant and the feedstock particles are typically reduced to a desired size suitable for handling in subsequent processing steps.
Among well-known methods used to convert cellulose to sugars is an acid hydrolysis process involving the use of steam and acid at a temperature, acid concentration and length of time sufficient to hydrolyze the cellulose to glucose (Grethlein, 1978, J. Appl. Chem. Biotechnol. 28:296-308).
An alternative method of cellulose hydrolysis is an acid prehydrolysis (or pre-treatment) followed by enzymatic hydrolysis. In this sequence, the cellulosic material is first pretreated using the acid hydrolysis process described above, but at milder temperatures, acid concentration and treatment time. This pretreatment process is thought to increase the accessibility of cellulose within the cellulosic fibers for subsequent enzymatic conversion steps, but results in little conversion of the cellulose to glucose itself. In the next step, the pretreated feedstock is adjusted to an appropriate temperature and pH and then submitted to enzymatic conversion by cellulase enzymes.
The hydrolysis of the cellulose, whether by acid or by cellulase enzymes, is followed by the fermentation of the sugar to ethanol, which is then recovered by distillation.
The efficient conversion of cellulose from cellulosic material into sugars and the subsequent fermentation of sugars to ethanol are faced with major challenges regarding commercially viability. In particular, the feedstock particles are often too large to be efficiently handled or processed. One desirable type of handling system that requires small particles is pumping. In dry processes, for particle size reduction, water is not added to the feedstock. Dry processes which include grinding, milling or crushing, require large amounts of power that adds to the cost of the overall process. Furthermore, dry processing to a small particle size suitable for pumping is unlikely to be successful for feedstocks having high or variable moisture contents. Some feedstocks containing 20% or higher moisture frequently blind the milling equipment, and it does not provide a sustainable or suitable size reduction. The alternatives are wet grinding processes or apparatus that make use of equipment such as using refiners or hydropulpers; however, wet grinding also requires costly equipment and high power usages. Furthermore, wet grinding produces a material that is very dilute and costly to handle and process.
A second problem with the conversion process is that the acid requirement in the pretreatment process is very high. Many feedstocks, such as straws and corn stover, contain a high native alkalinity that requires the addition of 0.5% to 6% w/w (of the feedstock) of sulfuric acid to achieve an efficient hydrolysis of the hemicellulose and exposure of the cellulose surface area. A significant amount of this acid serves to offset the alkalinity inherent within the feedstock. This high acid usage not only increases the cost of the process, but can also cause degradation of the xylose and other products during the pretreatment process.
WO 02/070753 (Griffin et. al.) describes a leaching process comprising contacting the feedstock with water for at least two minutes to leach out the salts, protein, and other impurities, followed by removal of the water and soluble compounds. The process of Griffin et. al. removes alkali from lignocellulosic feedstocks, thereby decreasing the acid requirement for pretreatment. Griffin requires particle size reduction, but the processes consume a high level of power and, in combination with the equipment required to carry out the leaching process, result in increased overall process costs.
The use of presses for dewatering biomass is known in the art. For example, U.S. Pat. No. 4,436,028 (Wilder) describes the use of a hammermill to greatly reduce particle size followed by a two-roll mill exerting severe pressure to decrease the moisture content of waste material. Similarly, U.S. Pat. No. 4,525,172 (Eriksson) teaches the dewatering of biomass using presses with sieving drums. However, these methods do not result in the grinding, shearing, or particle size reduction of the biomass during pressing or dewatering. This results in high capital and operating costs without achieving the necessary particle size reduction.
U.S. Pat. No. 4,543,881 (Anderson) discloses an apparatus for dewatering peat which includes an outer tubular roll and a smaller inner roll received in the outer roll. The smaller inner roll rotates so that its outer surface moves along the inner surface of the outer tubular roll, thereby compressing peat placed between the rolls to effect dewatering. By operating the rolls at different speeds, shear forces and compression forces act on the peat. However, the purpose of the roll compression apparatus is to dewater the peat and not to reduce particle size.
U.S. Pat. No. 2,828,081 (Collins) describes the use of roll presses to separate cork from phloem tissue on dry bark. A dried cork-rich fraction is passed through a differential speed roller mill that, through shearing action, breaks up the cork aggregates without substantially reducing the cork particle size. This process is not designed for size reduction of the particles of the material. As well, bark is not suitable for ethanol production.
In order to address the need for further particle size reduction of biomass, various approaches have been taken. For example, U.S. Pat. No. 6,036,818 (Odmark) describes a pulp dewatering device having two rolls through which the pulp is pressed (a roll press). As the pulp passes through the rolls, a doctor blade disintegrates and guides the pulp out of the press, and the pulp is further disintegrated by a screw disintegrator. U.S. Pat. No. 5,451,296 (Pikulin), teaches the use of a thickening unit (e.g. a twin roll press) to remove excess liquid from low consistency pulp. The resulting high consistency pulp is conveyed to a comminuting unit, such as a fluffer, to generate pulp particles of 10 mm or less. The roll presses used in either U.S. Pat. No. 6,036,818 or U.S. Pat. No. 5,451,296 do not convey grind, shear, or reduce the particle size of the pulp. Rather, additional equipment is required to accomplish particle size reduction, thus increasing both equipment and energy costs for the overall process.
U.S. Pat. No. 4,728,044 (Duill and Brummer) discloses a system for grinding and drying damp initial material. The starting material is pre-comminuted by a hammer mill while being dried with hot gas. Following further drying in a rising main, the material is further comminuted in the nip between the rollers of a high-pressure roll. The finished material emerges as dried and ground raw material. Although the process is suitable for the grinding and drying of materials such as raw cement meal, cement clinker, ore, coal and the like, the further processing of feedstock to produce ethanol is not addressed. None of the suitable feedstocks for ethanol production are mentioned.
U.S. Pat. No. 4,237,226 (Grethlein) describes milling of dry oak wood chips in a laboratory setting using a Wiley mill to produce a sawdust-like product. The ground chips pass through a screen of 60 mesh, then are slurried in water at a ratio of water to solids of 18.5 to 1 by weight prior to feeding the slurry to a continuous pretreatment reactor. The Wiley mill is not suited for use with fiber with over 20% moisture content, and exhibits high power consumption. Furthermore, there is no disclosure of commercial-scale equipment that may be used to carry out these processing steps.
Millett et al. (Biotechnol. & Bioeng. Symp. No. 6 (1976) 125-153) disclose several physical treatments for the preparation of feedstocks, including dry ball milling, wet ball milling and vibratory ball milling. The production of fine particles by dry ball milling adds substantially to the cost of the process, while wet ball milling for 72 hours increased the digestibility of cellulose by rumen bacteria. However, 72 hours is not a practical treatment time in a production process, and there is no mention of subsequent pretreatment or enzymatic hydrolysis. Vibratory ball milling of dry spruce and aspen chips for 30 minutes at 220° C. was found to increase the rate of enzymatic hydrolysis. However, this treatment adds considerable expense to the process.
U.S. Pat. No. 3,554,453 (Thale et al.) discloses an apparatus for shredding fibrous articles such as groundwood, compressed webs and flat pieces of sulfite and semi-chemical pulp. The apparatus contains a shredding roller and a holding roller, each with interdigitating toothed discs for shredding the fibrous material as it advances between the rollers. The action of the toothed discs on the rollers generates defibered material and does not result in pressing of the material.
U.S. Pat. No. 4,683,814 (Plovanich et al.) discloses an apparatus and a dry process for dewatering cellulosic biomass, which utilizes a pair of smooth opposed rolls operating at different speeds. Due to the differential roller speeds, the compressed biomass is heated, which results in additional moisture removal, and particle size reduction. Furthermore, moisture collects on the roll rotating at the higher rate and compressed material adheres to the roll rotating at the lower rate. This allows moisture to be collected from the roll rotating at the higher speed and compressed material to be collected from the roll rotating at the lower speed. Although the process provides an effective means for the dewatering of biomass, the process of Plovanich et al. only removes a minority of the alkalinity of the feedstock. Moreover, Plovanich et al. do not teach the production of a cellulosic feedstock having a particle size suitable for pumping.
The process for extracting sugar from sugar cane feedstocks is well known. This involves washing the sugar cane surface to remove impurities, coarsely chopping the stalks into smaller pieces, and crushing the sugar cane pieces in a series of roller mills to extract the juice. The juice is collected from the presses and further processed to produce sugar. The residue from the cane stock after juice extraction (bagasse) is usually burned at the mill.
In order for a continuous pretreatment of cellulosic feedstocks to be economically and commercially viable, the pretreatment system must be amenable to the pretreatment of a variety of feedstocks; the alkalinity of the feedstock must be reduced from its native levels, so as to decrease the acid requirements and degradation of sugar products by acid; and the feedstock particle size must be reduced, without requiring excessive power or capital equipment, such that the particles can be pumped in aqueous medium.
The development of such a system remains an important component of the overall process to convert cellulosic feedstocks to glucose and subsequently to ethanol.